An intra-oral single-section demineralization/remineralization model

This paper reviews the use of our intra-oral crown single-section model for the evaluation of in situ demineralization and remineralization. The model uses normal, healthy adults who are in need of a gold crown. A slot is placed in the working crown which can hold 3-4 single sections. A typical experiment would use an enamel lesion, root lesion, and sound root section which are characterized with polarized light microscopy and/or microradiography prior to insertion in the crown. After the experimental regimen, the sections are removed and re-characterized for any changes. The model has been used to evaluate mineral changes from the use of fluoride dentifrices and rinses, chewing gum, and food sequencing. The advantages of the model system are the before-and-after measurements on the same section, a natural plaque formation, interaction with saliva, episodic demineralization and remineralization, no bulky appliances, and the use of reasonable time frames. This model, therefore, reduces the artificiality of the in situ study to a great extent and appears to be a useful predictor of demineralization/remineralization interactions.

Fluoride-releasing core build-up materials and artificial caries

This study examined the effect of fluoride-releasing core build-up materials upon both enamel and root demineralization in an artificial caries system. Thirty extracted human molars were randomly assigned to three treatment groups consisting of (1) conventional composite core build-up paste, (2) fluoridated core build-up paste and (3) glass cermet. Teeth were thermocycled 350 times between 5 degrees C and 55 degrees C using 30-second dwell times. Acid-resistant varnish was placed on the specimens to within 1 mm of the restoration and the teeth were subjected to an artificial-caries challenge at pH 4.2 for 72 hours. Teeth were then sectioned and the thin sections photographed using a polarized light microscope. Areas of the artificial-caries lesions were measured using a sonic digitizing pad. Mean lesion area (in arbitrary units from the digitizing pad) on enamel for Group 1 was 3.76 +/- 0.54; Group 2 was 2.53 +/- 1.17 and Group 3 was 2.32 +/- 0.68. Mean lesion area on root for Group 1 was 3.04 +/- 0.54; Group 2 was 2.34 +/- 0.58 and Group 3 was 2.51 +/- 0.53. ANOVA with Duncan's multiple range test of contrast indicates a significant difference (P less than 0.001) on both enamel (35 +/- 3%) and root (20 +/- 3%) between the non-fluoridated product and the two fluoridated products. There were no significant differences between Group 2 and 3.

CLINICAL SIGNIFICANCE

This artificial caries study indicates that a fluoride-releasing core material may be of value in preventing demineralization of tooth structure adjacent to the restorative core material.

Effects of the XR-bonding system on in vitro caries

This study evaluated the effects of a dentin bonding system and glass ionomer liner on in vitro recurrent caries around resin composite restorations in dentin. The dentin adhesive significantly reduced the size of recurrent lesions. The glass ionomer liner, applied either to the axial wall or to the cavosurface margin, provided an even greater cariostatic effect. Both the adhesive and the liner reduced the size of cavity wall lesions. In addition, zones of caries inhibition were present around 75% of restorations lined with glass ionomer cement.

Effect of fluoride amalgams on artificial recurrent enamel and root caries

The present study examined the effect of F-containing amalgams on recurrent caries in vitro. Four amalgams were tested: A - Conventional amalgam; B - Amalgam A with 1% SnF2; C - Non-gamma-2 amalgam; D - Amalgam C with 1% SnF2. Twenty fillings from each amalgam were placed at the cementoenamel junction in sound extracted human teeth. The teeth were covered with varnish except for a 2-mm-wide zone around the fillings and immersed in separate vials containing dialyzed 15% gelatin gel, pH 4.2, for 17 days. Longitudinal sections through the created enamel and root lesions were examined using polarized light microscopy with distilled water as imbibition medium. Lesion depth was measured adjacent to the fillings and at the midpoint of the lesions. In the amalgam D group, 14 enamel lesions and 15 root lesions showed no demineralization in the area closest to the fillings, and mean lesion depth was significantly smaller than adjacent to the F-free amalgams in enamel as well as in root surfaces. In the root lesions, protection was most evident in amalgam D. At the midpoint of the lesions, depths were not statistically different between groups A-D. This study suggests that fluoride amalgams have anticaries properties sufficient to inhibit recurrent caries.

Artificial root caries in amalgam restorations: effect of light-cured fluoride releasing liners

The purpose of this study was to examine artificial root caries when light-cured fluoride releasing liners were placed under amalgam restorations. Class V preparations in extracted third molars were used with gingival margins on root surfaces. Ten restorations were used for each of the following groups: 1) Amalgam alone; 2) Two layers of copal varnish and amalgam; 3) Vitrabond, amalgam; 4) Timeline, amalgam; 5) XR Ionomer, amalgam. The teeth were thermocycled and artificial caries were created using a liquid system acidified to pH 4.20 containing 2.2 mM calcium and phosphate but without fluoride. The teeth were sectioned, polished and photographed using polarized light. Areas of recurrent caries were measured using a sonic digitizing pad. The data were analyzed using ANOVA and Duncan's Multiple Range Test. Areas for root lesions for the different groups were: 1) 2.17 +/- 0.35; 2) 1.90 +/- 0.40; 3) 1.30 +/- 0.18; 4) 1.77 +/- 0.28; 5) 1.50 +/- 0.33. Groups 3 and 5 were statistically different from groups 1 and 2, while no differences were observed for enamel lesions. The use of the photo-activated/fluoride releasing liners significantly reduced lesion area.

Effects of processed cheese on human plaque pH and demineralization and remineralization

This two-part study was undertaken to examine the effects of processed cheese on human plaque pH and de- and remineralization of enamel and root lesions in a human in situ caries model system. In the first part of the study the selected processed cheese (Kraft American Singles Processed Cheese Food) was eaten alone and followed by a 10% sucrose rinse after the acidogenicity of the plaque was demonstrated. A 10% sucrose rinse alone resulted in a mean minimum pH of 4.26. The cheese alone showed a mean minimum pF of 6.32 and cheese followed by sucrose resulted in a mean minimum pH of 6.48. The plaque pH of cheese eaten alone stayed at pH above 5.7 (the "safe for teeth" level). Cheese consumption also prevented the acid challenge when followed by sucrose. The second part of the study utilized the thin-sections of artificially created caries-like lesions on enamel and root, and sound root sections. One-month periods were used in a cross-over design to examine the effect of eating the cheese q.i.d. Polarized light microscopy was used to determine changes in the size of lesion areas. The addition of the processed cheese to the diet resulted in statistically significant reductions in enamel lesion size as well as a reduction in progression of root lesions. Lesions created on the sound root surfaces were approximately one-third the size of those created during the control period. This study indicates that processed cheese is hypoacidogenic, anti-acidogenic, and prevents demineralization as well as enhances remineralization.

Root hard-tissue demineralization rate measured by 125I absorptiometry: comparison with lesion-depth measurements

The aim of the present study was to compare demineralization of root hard tissue, monitored by 125I absorptiometry, with lesion-depth measurements under polarized light microscopy. The intact roots of ten human molars, which had not been exposed to the oral environment, were divided into 39 cementum/dentin blocks and exposed to a buffer solution of pH 4.5 containing 2.2 mmol/L calcium and inorganic phosphate. After demineralization for 3.5, 7, 14, and 21 days, transmission measurements by 125I absorptiometry were performed, and one block from each tooth was taken out of the solution for lesion-depth measurement. The results showed a high degree of correlation (r = 0.952) between lesion depth and change in transmission, with a more rapid increase initially in both variables. A linear relationship with the square root of time was found. Conversion of transmission data to lesion-depth data was possible when this caries model system was used on cementum dentin blocks.

Effects of a fluoride-releasing fissure sealant on artificial enamel caries

This study examined the effect of a fluoride-releasing fissure sealant upon enamel demineralization in an artificial-caries system. Twenty extracted human molars were randomly assigned to two treatment groups consisting of (1) conventional fissure sealant and (2) fluoride-releasing sealant. Specimens were acid-etched, rinsed, dried, sealant applied, and photocured for 20 seconds. Teeth were thermocycled 500x between 5 degrees C and 55 degrees C using 1 minute dwell times. Acid-resistant varnish was placed on the specimens to within 1 mm of the sealant and the teeth were subjected to an artificial-caries challenge at pH 4.2 for 62 hours. Teeth were then sectioned and the thin-sections photographed using a polarizing-light microscope. Areas of the artificial-caries lesions and their depths were measured using a sonic-digitizing pad. Mean lesion depths (in arbitrary units from the digitizing pads) were 4.17 +/- 0.95 for Group 1 and 1.22 +/- 0.61 for Group 2. Mean areas were 17.26 +/- 7.32 for Group 1 and 3.12 +/- 2.59 for Group 2. Students' t-tests indicated statistically significant differences (P less than 0.001) between groups for both measures. This artificial-caries study indicates that a fluoride-releasing fissure sealant substantially reduces the amount of enamel demineralization adjacent to the material.

Effects of fluoride on caries development and progression using intra-oral models

This paper reviews the use of intra-oral model systems to help elucidate the role of fluoride and its mechanism of action in caries prevention. The intra-oral models currently in use were found to be of three general types. The most widely used system has consisted of a removable appliance that relies on the use of dacron gauze or a recessed sample to enhance plaque formation. Similarly, the banding model of Ogaard requires the presence of orthodontic band material to produce a plaque accumulation niche for demineralization, while the crown single-section technique relies mainly on placement of the sections in plaque-retentive areas (below contact points). In general, the models may be used for the assessment of food cariogenicity, an evaluation of de- and re-mineralization, and measurement of fluoride incorporation into enamel or root substrates. On evaluation of lesion initiation and progression in vivo, it is apparent that few non-destructive in vivo techniques are available that offer the sensitivity of laboratory-based analysis. Thus, the use of intra-oral models that allow lesion formation and progression to occur in the oral environment, but can be measured with the sensitivity of in vitro techniques, has been extremely important. Although the magnitude of the fluoride dose, the longevity of fluoride in the oral environment, and the time required for remineralization are different from those found in vitro, it is apparent that the presence of fluoride in the aqueous phase is now thought to be of primary importance. Mechanistically, the presence of fluoride will both inhibit demineralization by acid and promote remineralization under more neutral conditions. Thus, one of fluoride's major contributions is to affect the rates of lesion formation and progression. It was concluded that low-concentration fluoride agents with a high frequency of application would best fulfill the above needs.

Human plaque pH responses to meals and the effects of chewing gum

Interproximal plaque pH responses to five different meals were investigated in this study. All meals were found to be acidogenic, with pH challenges lasting well over one hour. The effects of chewing one sorbitol and two different types of sucrose-containing gum for 20 minutes after the meal were examined. All three types of gum reversed the acid challenge of the meal and resulted in an interproximal pH level that is considered 'safe for teeth'. This study indicates that meals can be very acidogenic and that, in addition to normal preventive dental procedures, chewing gum for 20 minutes after meal consumption should be considered, to reduce the cariogenic challenge to the teeth.

An in vitro evaluation of the prevention of caries on overdenture abutments

Teeth prepared as overdenture abutments are susceptible to caries, and it has been shown that brushing by itself is not sufficient to prevent this process. This study evaluated the preventive effect of a remineralization gel which has a low fluoride concentration and compared its effects with those of a phosphate fluoride gel (Karigel), which has a much higher concentration of fluoride. Twenty extracted anterior teeth from patients aged 50 to 70 years were prepared as for overdenture abutments. Each tooth was sectioned into three fragments. An acidified gel system was used to produce artificial caries lesions on the occlusal and root surfaces of each fragment. One fragment of each tooth was treated with the remineralizing gel, the second fragment with a high-fluoride gel, and the third fragment served as the control. Ten teeth were removed at two weeks and again at four weeks, and were sectioned and prepared for histological examination. The depth of the lesions was measured from standardized photomicrographs by means of a sonic digitizer. The conclusions were: (1) Lesions on the occlusal tended to be deeper than those on the root surfaces at four weeks but not at two weeks; and (2) the high-fluoride gel was more protective than the low-fluoride remineralizing solution at both time periods on the occlusal but not on the root surface.

In vitro root caries progression measured by 125I absorptiometry: comparison with chemical analysis

Radiation from a 125I source and a non-image-forming detector was used for non-destructive measurements of root caries progression. Blocks were cut parallel to the cementum surface of unexposed human roots. These blocks were then individually demineralized in under-saturated calcium phosphate solutions over an 84-hour period. In order for the in vitro root surface demineralization to be followed, the changes in transmission (delta T) through the blocks were measured, by 125I absorptiometry, eight times during the course of the experiment. Chemical analyses of the calcium output (delta Ca) from the blocks into the demineralizing solutions were also performed, and the rate of demineralization (Vdem) was calculated from these values. The precision of 125I absorptiometry was calculated from 176 duplicate transmission measurements, and the coefficient of variation was found to be 0.20%. The correlation coefficient between delta T and total delta Ca for each of 22 cementum/dentin blocks ranged between r = 0.934 and r = 0.998. The progression of root hard-tissue lesions observed by these two methods and by the calculated Vdem was found to be proportional to the square and cubic roots of time. The study shows that 125I absorptiometry can be used for continuous non-destructive measurements of root hard-tissue demineralization in vitro.

An in vitro evaluation of artificial caries-like lesions on restored overdenture abutments

An acidified dialyzed gelatin gel system was used to determine the caries resistance of a variety of restorative materials used to obturate the canal orifice of overdenture abutment teeth. The restorative materials used were Tytin, Tytin + Copalite, P30 + Scotchbond, Fuji Ionomer-Type II, and Miracle Mix. Polarized light microscopy and microradiography were used to examine the caries-like lesions adjacent to the restorations. The lesions formed in the Fuji Ionomer-Type II and Miracle Mix groups appeared arrested at the wall adjacent to the restoration, and did not penetrate apically down the wall as did those associated with the other restorative materials. The mean depths of lesions adjacent to Fuji Ionomer-Type II and Miracle Mix restorations were significantly less than those of Tytin, Tytin + Copalite, or P30 + Scotchbond.

Effect of topical fluoride treatments on fluoride distribution during in vitro caries-like lesion formation

Tooth sections were treated in vitro for 4 min with APF, SnF2, or received no treatment. Each treatment group then received washes of KOH, (24 h), an inorganic solution (24 h), or DDH2O (2 min) and were placed into dialyzed 15% w/v gel (pH 4.3) containing 0.15 mM hydroxyapatite, but no fluoride (less than 0.02 ppm F). Adjacent nontreated sound enamel acted as the control. Lesion microdissection, after 2 weeks exposure to the acidified gel, revealed an inverse relationship between lesion body fluoride concentration and lesion depth. The lesion depth was smallest, and the lesion body fluoride concentration was greatest in both fluoride-treated groups after a 2-min DDH2O wash, a 24-hour inorganic wash, and a 24-hour KOH wash, respectively. These data support the theory that the progress of carious lesions is related to the fluoride concentration in the lesion and that the fluoride concentration in the lesion is related to the acquired fluoride concentration in sound enamel.

The use of saturated DCPD in remineralization of artificial caries lesions in vitro

Dicalcium phosphate dihydrate (DCPD) may play a significant role in the caries lesion since it is a stable calcium phosphate phase under acidic conditions. The reaction of DCPD and fluoride, forming fluorapatite (FAP), may provide a potentially promising treatment regimen for remineralization of caries lesions in vivo. The purpose of this study was to determine whether a two-step DCPD and inorganic wash with fluoride can remineralize artificial caries-like lesions in vitro. We used the single-section technique to facilitate quantitation of the same tissue before and after the experimental regimen. The two-step remineralizing treatment was repeated three times and consisted of a two-minute saturated DCPD treatment (pH 2.1) followed by a 24-hour inorganic wash. Lesion parameters were recorded before and after treatment by the taking of polarized light photomicrographs of each section after imbibition in several media. The changes in the tissue following treatment were expressed as a percent change in the area of the initial pre-treatment lesion. Significant reductions (p < 0.02) in lesion pore volume were observed in all aqueous media examined. In the lesions after imbibition in quinoline, remineralization was also apparent from the significant increase in the area of the dark zone following treatment. This two-step DCPD treatment appears to remineralize artificial caries-like lesions effectively, but additional work is needed to determine whether it affords any protection against subsequent cariogenic challenges.

Development of an intra-oral single-section remineralization model

An intra-oral model system has been developed which uses a single-section technique for before-and-after measurements on the same tissue. This model allows for placement of sections of enamel at both buccal and approximal sites. These sections may be sound or possess white-spot lesions. This allows for the evaluation of lesion initiation, lesion progression, and/or lesion remineralization. The sections are appropriate for measurement by polarized light microscopy, and, although not included in this study, they are also suitable for microradiographic evaluation. With ten human volunteers, the model system was evaluated by means of three rinse regimens in a cross-over design. The distilled water rinse showed the whole range of possible changes (e.g., demineralization, no change, and remineralization). More consistent remineralization was observed when a fluoride rinse or a remineralizing fluid was used. This model system should prove to be quite useful in evaluation of demineralization and remineralization phenomena in the oral environment.

In vitro caries-like lesion production by Streptococcus mutans and Actinomyces viscosus using sucrose and starch

We investigated the formation of caries-like lesions on root and enamel sections by S. mutans and A. viscosus when grown in four different carbohydrate substrates. The substrates were: sucrose, starch + alpha-amylase, and a combination of starch and sucrose with and without alpha-amylase. Twenty-four sections with exposed windows on both the root and enamel surfaces were exposed to the individual bacterial species in each of the four substrates for three weeks. At three weeks, the sections were removed and lesion depths measured. When grown in sucrose alone, S. mutans produced significantly deeper lesions in both root and enamel surfaces when compared with A. viscosus. However, S. mutans failed to produce lesions when grown in "limited" sucrose with starch added, whereas A. viscosus in this same substrate produced lesions in the enamel and root surfaces. A viscosus also produced either similar or significantly deeper lesions in both root and enamel surfaces, when compared with S. mutans grown in the two substrates with amylase. Thus, under these in vitro conditions, it was demonstrated that A. viscosus could utilize starch, whereas utilization of starch by S. mutans was alpha-amylase-dependent.

Effect of proteolytic enzymes on caries lesion formation in vitro

This study investigates enzyme degradation of collagen at various pH levels and is designed to obtain information pertaining to the effect of proteolytic enzymes on caries-like lesion formation and progression. One hundred and eight sections of human teeth with exposed areas on enamel and root face, were placed into an enzyme/buffer system maintained at pH levels of 4.0, 5.5 and 7.0. Periodically, throughout the experimental period, sections were removed and examined histologically for caries-like lesion formation. Results showed consistent surface erosion occurring on the root face only at a pH level of 5.5 in buffer containing enzymes. Sections placed in pH 5.5 buffer only, which served as controls, showed obvious lesion formation on the root face, but with histologically intact surfaces. This interaction occurring between enzymatic activity and acid demineralization suggests that enzymes may have a contributory effect on caries lesion formation in root surfaces.

Fluoride distribution in enamel after in vitro caries-like lesion formation

Using a microdissection technique to obtain samples from carious and adjacent sound enamel, we compared the fluoride concentrations in surface and subsurface enamel. Undialyzed and dialyzed gels were used to create lesions with and without negatively birefringent surface zones, respectively. The fluoride concentration in the surface enamel of the lesions was always greater than that in the corresponding sound enamel, as was the subsurface fluoride concentrations of caries lesions compared with adjacent sound enamel at the same depth.

Natural root caries: a histologic and microradiographic evaluation

The purpose of this study was to obtain information about the detailed histopathology of naturally occurring root caries. Fifty extracted human teeth exhibiting some degree of root caries were sectioned longitudinally and examined with transmitted light, polarized light and microradiography. The occurrence of the histological zones of dentinal caries was tabulated and revealed a lesion body in all cases. Light microscopy showed the same basic features as microradiography and was most useful when the sections were imbibed in quinoline. A structureless area below the main body of the lesion was observed in 77% of the cases when using quinoline. This fluid more clearly defined the extent of the lesion and may show a "phenolic reaction" when using polarized light microscopy. Partial radiopaque surface layers were observed in almost 80% of the sections. This study has described the detailed histopathology of root caries, and aids in the development of model systems to evaluate this emerging dental health problem.

Depth of penetration of in vitro root surface lesions

An acidified gel system was used to produce artificial caries lesions in root surfaces and in enamel. The teeth were exposed to the gel system for two weeks and four weeks, after which the lesions were examined by polarized light microscopy and photographed by standardized techniques. The depths of penetration of the lesions were measured from these standardized photomicrographs. The lesions in root surfaces were deeper than those in enamel for all teeth. After two but not four weeks of exposure in the gel system, the lesions in the root surfaces of anterior teeth were deeper than those in pre-molars.

Histopathology of the initial lesion of the root surface: an in vitro study

An acidified gel system was used to produce artificial caries lesions in root surfaces. Radiopaque surface layers were found with 47% of the lesions. Demineralized bodies of the lesion and mineralized zones of banding were seen in all lesions when examined by microradiography. Cementum and dentin seemed to respond to the acid attack in a similar manner. Although artificial root lesions did not simulate natural root caries entirely, the acidified gel system resulted in a reproducible method that might give insight into initial lesion formation on root surfaces. This technique may be used for studying the physico-chemical process involved in the demineralization of root surfaces and for testing the effects of various agents in the prevention of root caries.